But, according to surprising new evidence, pollution causes more deaths from heart disease than lung disease, and has even been implicated in birth defects and sudden infant death syndrome (SIDS).The air we breathe is now so polluted that it kills three million of us around the world every year. In the United States alone, it kills 100,000 people a year, which is more than the death toll due to traffic accidents. Of course, the phrase 'death from air pollution' never appears on the death certificate; instead, the cause of death is put down to agitated asthma, bronchitis, emphysema, or some other lung-related condition-which, neverthe-less, have all been directly linked to air pollution.
Even more startling, however, is the evidence that exposure to air pollution appears to affect the heart as well as the lungs, and can-even at low levels-increase the risk of a fatal heart attack or stroke. Indeed, contrary to what you might expect, studies show that air pollution poses a greater risk of death from heart disease than from any respiratory ailment.
Air pollution and the heart
Earlier this year, a major US study published in a prestigious peer-review journal provided some of the most compelling evidence yet that air pollution increases the risk of heart disease (N Engl J Med, 2007; 356: 447-58).
Previous studies have found that high levels of air pollution are associated with death and hospital-ization for heart disease, but the present study-among the biggest of its kind-discovered that "the magnitude of health effects may be larger than previously recognized."
The American researchers used information from nearly 66,000 postmenopausal women, aged 50 to 79, living in 36 cities across the nation and participating in the Women's Health Initiative (WHI) Observational Study. None of the women had heart disease at the beginning of the study but, nine years on, 1816 of them had suffered a heart attack or stroke, undergone heart bypass surgery or died because of cardiovascular causes.
The researchers linked this information with data on the air quality around each woman's home, and discovered that those living in the most polluted areas had the highest risk of developing cardio-vascular disease and of dying from it. Higher, long-term exposure to air pollution was also linked to an increased risk for developing heart disease.
An interesting finding was that only fine particulate air pollution-such as from vehicle exhausts, coal-fired power plants and other industrial sources-was associated with an increased risk. Other community pollutants, including sulphur dioxide, nitrogen dioxide, carbon monoxide and ozone, were not linked with an increased risk of heart disease.
But perhaps most important of all, this new WHI study has confirm-ed a stronger statistical association between fine-particle air pollution and death from coronary heart disease than was found in the earlier studies.
A study by the American Cancer Society, for example, found that each 10-unit increase in the level of fine particulate matter (PM) in the air [known as PM2.5 because the particles measure less than 2.5 micrometres (mcm) in diameter] increased the risk of dying from heart disease by 12 per cent, higher than the risk of death from respiratory causes (Circulation, 2004; 109: 71-7). Similarly, the Harvard Six Cities study reported a 19-per-cent increased risk of death due to cardiovascular causes for every 10-microgram (mcg) increase in fine-particle concentration per 1 m3 of air (N Engl J Med, 1993; 329: 1753-9).
This latest WHI study, however, found that the risk increased by considerably more than that-in fact, a massive 76 per cent for each 10-mcg rise-an alarming discovery given that the average particulate levels for the 36 cities ranged from 3.4 mcg/m3 (in Honolulu, HI) to 28.3 mcg/m3 (in Riverside, CA).
Are women at greater risk?
According to environmental epi-demiologist Douglas Dockery, of theHarvard School of Public Health, it is now clear that fine-particle air pollutants pose a unique risk to health, although why this should be is not so clear. "It may be their chemical composition, their size, or their ability to transport other pollutants deep into the lungs," he says. "There is a lot of research going on right now attempting to figure this out" (see box, page 6).
However, what we do know is that some people appear to be more susceptible to the cardiovascular risks of air pollution than others.
In their commentary on the WHI study, Dockery and fellow Harvard expert Peter Stone point out that women are not the same as men when it comes to heart disease: "Women's coronary arteries are smaller in size and tend to harbour more diffuse atherosclerosis than do men's arteries, and women's microvessels appear to be more frequently dysfunctional than those of men." They suggest that gender, while it may not define susceptibility to air pollution, "may be an indicator of an underlying cardiac substrate that puts women at increased risk" (N Engl J Med, 2007; 356: 511-3).
So, this may account for the particularly strong association between air pollution and death from heart disease reported in the all-women WHI study. Other studies that included both men and women have also found greater cardiovascular effects of particulate air pollution in women than in men-especially older women (Environ Health Perspect, 2005; 113: 201-6, 1723-9).
Indeed, one of those studies-a 22-year follow-up of non-smoking white adults in California-showed an association between an increased risk of fatal heart disease and rising levels of fine-particle air pollution only in women (Environ Health Perspect, 2005; 113: 1723-9). The researchers suggested that particulates are deposited differently-and perhaps more harmfully-in women's lungs compared with men's (Environ Health Perspect, 2005; 113: A836-7).
Those who are elderly, diabetic and have underlying heart or lung disease may also be more vulnerable to the adverse effects of air pollution (Circulation, 2004; 109: 2655-71).
Thus, it appears that air pollution needs to be taken more seriously asa risk factor for heart disease in adults-but it isn't just grown-ups who are at risk. New evidence shows that the harmful effects of dirty air can extend even into the womb, and cause damage to the heart of the developing fetus.
Research carried out by the University of California at Los Angeles' (UCLA) School of Public Health and the California Birth Defects Monitoring Program (CBCMP) revealed that pregnant women exposed to high levels of ozone and carbon monoxide may be up to three times more likely to have a baby with a ventricular septal defect (a hole in the wall separating the two pumping chambers of the heart), as well as other valvular and aortic congenital defects.
The risk was highest when the women were exposed to pollutants during their second month of pregnancy, when the fetal heart and other organs are just beginning to develop.
But most worrying is the fact that virtually the entire study area, which included the Los Angeles, Orange, San Bernardino and Riverside Counties in California, met the federal standards for carbon monox-ide levels and were compliant with ozone requirements (Am J Epidemiol, 2002; 155: 17-25).
Another study, involving seven counties in Texas, also found a link between air quality and risk of congenital heart defects. Although levels of carbon monoxide were lower than in California, the researchers found that women exposed to the highest levels of this pollutant were twice as likely to give birth to babies with a heart defect than those exposed to the lowest levels. Particulate matter and sulphur dioxide were also associated with an increased risk of heart defects, and there is even evidence suggesting that air pollution may influence the risk of oral clefts (Am J Epidemiol, 2005; 162: 238-52).
Yet more evidence comes from Central Europe, where researchers have observed a connection between communities with high levels of air pollution and increased rates of heart and other birth defects (Acta Chir Plast, 1998; 40: 112-4; Mutat Res, 1993; 289: 145-55).
Clearly, there is something in the air that can disrupt normal fetal development.
The evidence so far
These studies are all part of a growing body of literature from around the world indicating that polluted air is taking a greater toll than was previously believed on infants and unborn babies. In addition to birth abnormalities, more than a dozen studies carried out in the US, Brazil, Mexico, China andthe Czech Republic have linked air pollution to low birth weight, premature birth, stillbirth and infant death (Am J Epidemiol, 2002; 155: 17-25).
The studies differ on which pollu-tants are of most concern. Some implicate gases, others blame particles, and yet others point the finger at both. But the general results of the studies are consistent: the higher the level of pollution, the higher the risk to babies.
One of the most significant findings is that outdoor air pollution seems to be particularly related to cot death, or sudden infant death syndrome (SIDS). The pollutant to blame appears to be PM10, partic-ulate matter of less than 10 mcm in diameter, found in vehicle exhaust fumes.
In one international study, scien-tists reported a link between PM10 and 16 per cent of unexplained deaths among babies of normalbirth weight. They suggested that pollution above 12 mcg/1 m3 of air "contributes in a substantial way to postneonatal infant mortality".
In that study, the average all-cause death rate was 236.8 deaths per 100,000 infants, with 14.7 per 100,000 attributed to PM10 pollu-tion. In cases of SIDS, the figure was 11.7 per 100,000 (Environmental Health: A Global Access Science Source, 2004; 3: 4).
Similarly, a study by America's Environmental Protection Agency found that SIDS was 26 per cent more likely in infants exposed to the highest levels of PM10 (Environ Health Perspect, 1997; 105: 608-12). According to an analysis of the data by non-profit research organization The Environmental Working Group (EWG), this translates to 500 SIDS cases each year attributable to air-borne particle pollution in the US.
However, a Canadian study that looked into the relationship between SIDS and air pollution found that gases (sulphur dioxide, nitrogen dioxide and carbon monoxide) rather than particles were most strongly associated with the incidence of SIDS (Pediatrics, 2004; 113: e628-31).
While more research is needed to establish which pollutants are the most harmful, what's already clear is that something needs to be done now to protect those at risk.
WHO says what?
There are a number of guidelines and standards covering outdoor air pollution, including the global air-quality guidelines recently issued by the World Health Organization (WHO; see http://whqlibdoc.who. int/hq/2006/WHO_SDE_PHE_OEH_06.02_eng.pdf). This document sets levels for particulate matter (PM), ozone, nitrogen dioxide and sulphur dioxide. The WHO maintains that if its standards were to be met for just one of these pollutants-say, PM-then deaths due to air pollution in cities could be reduced by as much as 15 per cent (Lancet, 2006; 368: 1302).
But a key question is whether there are any concentrations at which air pollution will have no effect on public health (Lancet, 2002; 360: 1233-42).
So far, the WHO notes that, at least for PM, no such thresholdhas yet been identified. In fact, adverse effects are seen at levels that are not much greater than the background concentrations of PM pollution in both the US and Western Europe which, for particles less than 2.5 mcm (PM2.5), is around 3-5 mcg/m3 of air.
"It is unlikely," says the WHO, "that any standard or guideline value will lead to complete protection for every individual against all possible adverse health effects."
This means it's up to all of us to guard ourselves against the very air we breathe.
Joanna Evans, with additional reporting by Bryan Hubbard
The polluted heart
Scientists are not certain how particulate air pollution increases the risk ofheart disease, but several mechanisms have been proposed.
One is that inhaling the particles causes oxidative stress and inflammation not only in the lungs, but throughout the entire body, leading to dysfunction of the autonomic system (which regulates heart rate and blood pressure), blood-vessel damage and atherosclerosis (hardening of the arteries). Studies show that a buildup of atherosclerotic plaque is higher in communities that have greater concentrations of fine-particle pollution (N Engl J Med, 2007; 356: 511-3).
Indeed, a study in mice-and so the findings may not necessarily apply to humans-showed a clear causal link between air pollution and atherosclerosis (JAMA, 2005; 294: 3003-10).
Another possibility is that exposure to air pollution induces changes inblood composition-with potentially serious effects on the heart. For example, particulate matter has been shown to lead to rapid and significant increases in fibrinogen and blood coagulation factors, well-established risk factors for heart attack and stroke (Circulation, 2004; 109: 2655-71).
That something in the air
Air pollution is a catch-all term that embraces a range of pollutants, each generated from a different source, and each with a potential to harm you, depending upon your health profile.
Source: fuel combustion from road traffic, industrial processing, agriculture and wood-burning
- Health hazards: exacerbation of respiratory problems such as asthma, bronchitis, emphysema and pneumonia; a cause of cardiovascular disease and sudden infant death syndrome (SIDS).
- Polycyclic aromatic hydrocarbons (PAHs)
Source: motor vehicles, industrial processes and other processes involving incomplete combustion of organic compounds
Health hazards: cardiorespiratory diseases related to air pollution; cancer.
Source: ozone pollution, or smog, occurs at ground level when it mixes
with nitrogen dioxide and volatile organic compounds (VOCs)-from power stations, cars, industrial plants and household activities-reacting with sunlight
- Health hazards: irritates the respiratory tract and eyes; causes breathing difficulties such as shortness of breath, coughing and wheezing (short-term exposure). Children are especially vulnerable as their smaller lungs are less able to cope with higher ozone levels. High levels of ozone may also cause lasting damage to the developing fetus.
- Carbon monoxide, sulphur dioxide, hydrogen chloride, hydrogen fluoride, benzene and ammonia
Source: various industrial, chemical and agricultural processes
Health hazards: respiratory and cardiovascular problems; increased risk of cancer; reduced birth weight of a newborn if the mother is exposed to excessive amounts during pregnancy (carbon monoxide).
Minimizing your risk
How can we protect ourselves from the health risks of breathing dirty air? First, become a student of your environment. Do you live near a busy road or motorway? Is there a farm within a few miles? Is your home or office within a 20-mile radius of an industrial processing plant?
You also need to become a student of yourself. What is your health like? Areyou sensitive to airborne pollutants? What is your family history? These factors will help determine whether air pollution is a major factor in your own state of health. But whatever the prognosis, there's plenty you can do to offset its effects.
- Pump up your intake of antioxidants. Combat the oxidative stress causedby air pollutants on your system by ensuring a plentiful supply of free-radical scavengers. Vitamins C and E are particularly effective against nitrogen dioxide and ozone, respectively, in lung disease (Ann NY Acad Sci, 1992; 669: 141-55). However, for good all-round benefits, take the major antioxidants at these suggested daily dosages for adults.
- Vitamin A: 2660 IU for women; 3330 IU for men
- Vitamin C: at least 1 g
- Vitamin E: 400 IU
- Selenium: up to 200 mcg.
These dosages are all well within safety limits. However, there is a small risk in some people who may react to too high a dosage, so make sure you first consult a qualified and experienced practitioner.
- Purify your environment with ionizers. Install these devices in your car and house, or even wear a personal ionizer around your neck. An ionizer shoots out negatively charged particles-'negative ions'-into the air that attach to pollutants, causing them to 'drop out' of the air.
- Stop smoking. Evidence suggests that smoking can interact with air pollution to increase the risk of death due to arrhythmias, heart failure and cardiac arrest (Circulation, 2004; 109: 2655-71).
- Know what's out there. To keep tabs on levels of pollution in your area, contact your local air-quality monitoring service (in the UK: The Air Quality Archive: www.airquality.co.uk/archive/index.php; in the US: Environmental Protection Agency: www.epa.gov./tri/). If pollution levels are high:
- Stay indoors as much as you can, where the levels of many pollutants are lower than outdoors
- If you must go outside, limit outdoor activities to before noon or wait until after sunset, especially when smog levels are high.
- Stay away from high traffic areas and avoid outdoor activities near these areas at all times. Find a park for your children to play where there is little surrounding traffic.
- Don't exercise or exert yourself outdoors when air-quality reports indicate unhealthy conditions. The faster you breathe, the more pollution you take into your lungs.
- Do your bit. Traffic pollution is the biggest offender, so avoid using the car as much as you can. If you have to drive, make sure your car is as 'green' as possible by keeping it regularly tuned-a petrol-efficient car can considerably reduce noxious emissions. Also, keep your speed down. A slower car burns less fuel and, thus, produces less exhaust. For more information on becoming a greener motorist, visit the Environmental Transport Association's website
at www.eta.co.uk, or see www.greenercars.com.
- Don't live near a main road. If you're moving house, find one that isn't in close proximity to a busy road. One large-scale study conducted in England and Wales found that men and women living within 200 metres of a main road had a 5-per-cent increased risk of stroke compared with those living 1000 metres or more away (Stroke, 2003; 34: 2776-80).
The countryside trap
Many of us believe they can escape polluted cities by moving to the greener pastures of the countryside. Sadly, the truth is, you're safer in the city. Rural areas suffer more days of air pollution than urban areas.
Levels of ozone can be 20-40 per cent higher in the country. This is because the greater concentrations of road traffic in cities and towns create more pollutants that 'scrub out' the newly formed ozone. Ironically, it is the cleaner air in the countryside that allows ozone to linger and accumulate.
In 2004, urban areas recorded an average of 22 days of moderate and higher air pollution-but the figure was twice as high for rural areas. The same trend was seen in 2003, when cities recorded an average of 50 days of moderate or higher air pollution compared with 61 days in the countryside.
Although ozone levels, especially during a long hot summer, may be the main cause of this turnabout, it's also important to remember that agricultureis among the chief culprits for spreading air pollution. In addition, industrial conglomerates also tend to locate their processing plants in rural areas.
Despite this, large urban areas are still far worse for nitrogen dioxide fromcar exhausts. Smaller towns located near major motorways are also likely to suffer the fallout from heavy road traffic.
The same pattern can be seen on the emissions map for volatile organic compounds (VOCs) and carbon monoxide, both of which are major components of exhaust fumes. Interestingly, although lead emissions show a similar geographical trend, overall concentrations are lower, possibly because of the controls on using leaded petrol. High sulphur-dioxide emissions are not as widespread as nitrogen dioxide, tending to be focused more on areas supporting heavy industry.